Memories may be stored on your DNA

REMEMBER
your first kiss? Experiments in mice suggest that patterns of chemical
“caps” on our DNA may be responsible for preserving such memories.

To
remember a particular event, a specific sequence of neurons must fire
at just the right time. For this to happen, neurons must be connected
in a certain way by chemical junctions called synapses. But how they
last over decades, given that proteins in the brain, including those
that form synapses, are destroyed and replaced constantly, is a mystery.

Now Courtney Miller and David Sweatt
of the University of Alabama in Birmingham say that long-term memories
may be preserved by a process called DNA methylation – the addition of
chemical caps called methyl groups onto our DNA.

Many
genes are already coated with methyl groups. When a cell divides, this
“cellular memory” is passed on and tells the new cell what type it is –
a kidney cell, for example. Miller and Sweatt argue that in neurons,
methyl groups also help to control the exact pattern of protein
expression needed to maintain the synapses that make up memories.

They
started by looking at short-term memories. When caged mice are given a
small electric shock, they normally freeze in fear when returned to the
cage. However, then injecting them with a drug to inhibit methylation
seemed to erase any memory of the shock. The researchers also showed
that in untreated mice, gene methylation changed rapidly in the
hippocampus region of the brain for an hour following the shock. But a
day later, it had returned to normal, suggesting that methylation was
involved in creating short-term memories in the hippocampus (Neuron, DOI: 10.1016/j.neuron.2007.02.022).

To
see whether methylation plays a part in the formation of long-term
memories, Miller and Sweatt repeated the experiment, this time looking
at the uppermost layers of the brain, called the cortex.

They found that a day after the shock, methyl groups were being removed from a gene called calcineurin
and added to another gene. Because the exact pattern of methylation
eventually stabilised and then stayed constant for seven days, when the
experiment ended, the researchers say the methyl changes may be
anchoring the memory of the shock into long-term memory, not just
controlling a process involved in memory formation.

“We
think we’re seeing short-term memories forming in the hippocampus and
slowly turning into long-term memories in the cortex,” says Miller, who
presented the results last week at the Society for Neuroscience meeting in Washington DC.

“The
cool idea here is that the brain could be borrowing a form of cellular
memory from developmental biology to use for what we think of as
memory,” says Marcelo Wood, who researches long-term memory at the
University of California, Irvine.